The present invention concerns a method of forming a high density recycle gas for a reforming process.
Catalytic reforming is well known in the petroleum industry. It involves treating naphtha fractions to improve the octane rating by producing aromatics. The hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of acylic hydrocarbons to aromatics, dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butanes. Hydrocracking reactions should be minimized during reforming as they decrease both the yield of products in the gasoline boiling range and the yield of hydrogen.
Because of the demand for high octane gasoline for use in motor fuels, extensive research is being devoted to developing improved reforming catalysts and catalytic reforming processes. Catalysts for reforming processes must be able to produce high yields of liquid products in the gasoline boiling range and low yields of light gaseous hydrocarbons. The catalysts should possess good activity in order that low temperatures can be used to produce a quality product. The catalysts should also either possess good stability, in order that the activity and selectivity characteristics can be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalysts comprising platinum, for example, platinum and rhenium supported or alumina, are widely used for the reforming of naphthas.
Some have proposed the use of certain molecular sieves such as X and Y zeolites, which have pores large enough for hydrocarbons in the gasoline boiling range to pass through. However, reforming catalysts based upon these molecular sieves have not been commercially successful.
In conventional reforming, the hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of about 450.degree. C. to 550.degree. C. and pressures of about 50 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The catalysts hitherto used have given fairly satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction and produce excellent results with C.sub.6 -C.sub.8 paraffins.
A characteristic of reforming reactions is that there is a temperature drop as the reaction takes place which is proportional to the overall heat of reaction, proportional to conversion, and inversely proportional to the overall heat capacity per unit of reactants. Therefore, in those cases where the reformer is operated to produce a high yield of aromatics it would be advantageous to add a heat carrier to the reformer feed, so that the overall heat capacity per unit of reactant gas would increase and the temperature drop for a targeted maximum conversion would decrease. Decreasing the temperature drop would allow operation with a lower reactor inlet temperature.